Free-Space Laser Communications (Lasercom) requires high-precision pointing control to track a remote laser communication terminal or lasercom terminal so that the two terminals can maintain a laser communications link between them. The pointing, acquisition, and tracking system of the laser communication terminal typically uses two-axis gimbals for coarse pointing control to an accuracy of several hundred micro-radians and a Fast Steering Mirror (FSM) for fine track control to maintain pointing to within micro-radians. A major source of LOS error may be from base jitter or motion of the laser communication terminal that may be created by structure vibration of the host spacecraft. The base jitter may be caused by other moving parts of the spacecraft, such as rotating wheels, thrusters or similar devices for maintaining stability and position of the spacecraft.
The FSM position control is typically based on the feedback of a measured LOS error (also called track error), representing a measure of LOS jitter of a received laser signal from the remote laser communication terminal. The LOS error measurement may be made with an optical receiver responding to the laser beam from the remote terminal. An FSM control algorithm may calculate a control signal to drive the FSM to null or substantially mitigate the jitter. However, in the condition of power fade where the signal power of the received laser beam or signal is reduced because of interference, noise or other causes, the LOS error measurement may become unreliable because of excessive noises.